Abrasion-resistant protective laminates

ABSTRACT

Multi-layer sheet materials useful as protective laminates for application to substrates, such as photographic elements, which require the protection of an abrasion-resistant material, are comprised of a strippable carrier, an abrasion-resistant layer which overlies the carrier, a structural interlayer which overlies the abrasion-resistant layer, and an adhesive layer which overlies the structural interlayer. The structural interlayer is very thin in relation to the thickness of the carrier, and is permanently bonded to one surface of the abrasion-resistant layer, whereas the carrier is releasably bonded to the other surface of the abrasion-resistant layer so as to permit it to be stripped away after it has served its purpose. A process for manufacturing the multi-layer sheet material comprises the steps of advancing the carrier in web form through a coating station at which it is coated with a layer of liquid composition that is curable to a solid form; laminating the structural interlayer to the coated surface of the carrier; curing the liquid layer in situ to a solid form to thereby form an abrasion-resistant layer; and applying an adhesive layer over the surface of the structural interlayer.

This is a continuation of application Ser. No. 210,280, filed June 23,1988 now abandoned.

FIELD OF THE INVENTION

This invention relates in general to protective laminates and inparticular to multi-layer sheet materials useful as protective laminatesfor application to substrates requiring protection with anabrasion-resistant material. More specifically, this invention relatesto novel multi-layer sheet materials which include a strippable carrierwhich facilitates application to a substrate of an extremely thin andvery smooth abrasion-resistant protective laminate. It also relates to anew and improved method for manufacture of such multi-layer sheetmaterials.

BACKGROUND OF THE INVENTION

It is well known to employ protective laminates to provide anabrasion-resistant surface on substrates that are subject to abrasiondamage. Among the many uses for such protective laminates, one whichimposes especially stringent requirements is use in the photographicfield. Many photographic products are amenable to significantimprovement in their performance through use of such abrasion-resistantlaminates, for example, photographic color prints, graphic arts films,X-ray films, motion picture films, microfilms and microfiche.Photosensitive elements known in the art as "phototools" have an acuteneed for the type of protection against abrasion and scratches that canbe provided by a protective laminate, and impose especially demandingconstraints. Such elements are used, for example, in the preparation oflithographic printing plates and in the utilization of photoresists inthe production of printed circuits. The phototool is composed of asupport with a high degree of dimensional stability and one or morelayers of a photosensitive material, such as a silver halide emulsion, aphotopolymer composition or a diazo composition coated on the support.The photosensitive layer has a relatively soft surface and is therebysubject to scratching, abrasion and other damage in use which severelydetracts from its capabilities. Moreover, the optical and mechanicalrequirements are very stringent, so that a useful protective laminatemust provide the desired protection without interfering with the abilityof the phototool to meet these exacting requirements.

Many attempts have been made heretofore to produce protective laminatesthat are useful with substrates needing protection against abrasion,scratches and the like, including phototools and other photographicmaterials, as well as glass, plastics, metals and many types of fragilecoatings. Protective laminates particularly intended for use withphototools are described in Fulwiler, U.S. Pat. No. 4,077,830, issuedMar. 7, 1978; while those particularly intended for use withphotographic color prints are described in Segel, U.S. Pat. No.4,378,392 issued Mar. 29, 1983, and Miller, U.S. Pat. No. 4,581,267,issued Apr. 8, 1986. The protective laminates of Eshleman, U.S. Pat. No.4,337,107 issued June 29, 1982 are described as being useful in theprotection of a very wide range of substrates, including photographicelements and many other materials.

While the protective laminates of the prior art are useful in manyapplications, they suffer from serious deficiencies which have hinderedmore widespread commercial application. Thus, for example, they arefrequently too thick and thereby subject to causing optical distortionto too great an extent to be successfully employed in the photographicfield. Another common problem is their ability to provide a protectivesurface with as high a degree of resistance to abrasion and scratchingas is needed. Yet another concern is the tendency of protectivelaminates to cause excessive curling of the photographic element towhich they are applied. A still further problem is the great difficultyin manufacturing a protective laminate which is extremely thin, and inapplying such laminate in a wrinkle-free state, because of the manydifficulties that arise in conveying and handling extremely thin webmaterials.

It is toward the objectives of providing a new and improved protectivelaminate which overcomes the deficiencies of the prior art, and ofproviding a commercially practical method for the manufacture of suchlaminate, that the present invention is directed.

SUMMARY OF THE INVENTION

In accordance with this invention, a multi-layer sheet material isutilized to form an abrasion-resistant protective laminate on thesurface of a substrate requiring protection from abrasion. Themulti-layer sheet material comprises a strippable carrier comprised of arelatively thick layer of a flexible material, an abrasion-resistantlayer which overlies the carrier and is releasably bonded thereto, avery thin structural interlayer formed of flexible material overlyingthe abrasion-resistant layer, and an adhesive layer overlying thestructural interlayer. The structural interlayer is very thin inrelation to the thickness of the carrier, and is permanently bonded toone surface of the abrasion-resistant layer, whereas the carrier is onlytemporarily bonded to the abrasion-resistant layer since it is intendedthat the carrier be stripped from the remainder of the multi-layer sheetmaterial by the user. In some embodiments of the invention, themulti-layer sheet material also includes a release sheet which coversthe adhesive layer so as to permit the sheet material to be wound inroll form without adjacent convolutions of the roll sticking together.

To protect a substrate, such as, for example, a phototool or otherphotographic element, from abrasion, the adhesive surface of the sheetmaterial is pressed, after removing any release sheet, into conformingcontact with the substrate and is bonded thereto. Thereafter, thecarrier is stripped away so as to expose the underlyingabrasion-resistant layer. By virtue of the excellent abrasion-resistantand scratch-resistant properties of the abrasion-resistant layer, thesubstrate is provided with a durable protective surface, whereas byvirtue of the extreme thinness of the laminate applied, there is nosignificant adverse effect on optical characteristics or otherproperties. Since the carrier is strippable, the layers which remain onthe substrate are only the adhesive layer, the interlayer and theabrasion-resistant layer and, since each of these can be extremely thin,the overall thickness of the protective laminate is much less than thatof many of the prior art laminates.

A simple and efficient method for manufacturing the multi-layer sheetmaterial of this invention comprises the steps of advancing the carrierin web form through a coating station at which it is coated with a layerof liquid composition that is curable to a solid form; laminating thestructural interlayer to the coated surface of the carrier usingsuitable laminating means such as pressure rollers; curing the liquidlayer to a solid form to thereby form an abrasion-resistant layer; andapplying an adhesive layer over the surface of the structuralinterlayer. Additional steps may also be employed such as, for example,the steps of securing a release sheet over the exposed surface of theadhesive layer and winding the sheet material into roll form forconvenient handling in use. Since the carrier is relatively thick, it iseasily conveyed through the coating station where the curable liquidcomposition is applied; whereas the very thin structural interlayer doesnot have to be coated with the curable liquid composition, but is merelysubjected to a wet laminating step. In this way, the seriousdifficulties ordinarily encountered in conveying and coating a very thinweb without distortion, wrinkling, or damage are effectively avoided,and the manufacturing process is one which is easily and convenientlyconducted on a commercial scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a multi-layer sheet material of thisinvention composed of a strippable carrier, an abrasion-resistant layer,a structural interlayer, and an adhesive layer.

FIG. 2 is a cross-sectional view of the sheet material of FIG. 1additionally comprising a release sheet over the adhesive layer.

FIG. 3 is a cross-sectional view of the sheet material of FIG. 1additionally comprising a release coating between the carrier and theabrasion-resistant layer.

FIG. 4 is a cross-sectional view of the sheet material of FIG. 1additionally comprising a release coating on the underside of thecarrier.

DETAILED DESCRIPTION OF THE INVENTION

The multi-layer sheet material of this invention is useful in theprotection of a wide variety of different substrates and can bemanufactured from a wide variety of different materials. The optimummaterials and optimum layer thicknesses will depend in part on the enduse requirements of the laminate, and the particular materials chosenwill in turn require appropriate modification of the manufacturingprocess. Key features of the multi-layer sheet material include therequirement that the structural interlayer be very thin in relation tothe thickness of the carrier, and that the abrasion-resistant layer bepermanently bonded to the structural interlayer and releasably bonded tothe carrier.

The method of manufacture described herein is also capable of widespreadvariation, depending on the particular choice of materials andtechniques. Key features of the method include the utilization of twocoating steps and one wet laminating step--whereby the very thinstructural interlayer is incorporated into the multi-layer sheetmaterial by wet lamination and the curable liquid composition andadhesive composition are incorporated by coating processes--and theformation of the abrasion-resistant layer by an in situ curing process.

In the multi-layer sheet material of this invention, the strippablecarrier performs several important functions. Thus, for example, itserves as the support on which the curable coating composition iscoated. Because it is relatively thick, it is easily transported, so asto facilitate the coating step, and it is able to withstand substantialtension so as to resist curling or other distortion caused by theshrinkage that is normally associated with the curing of a curablecoating composition. Because the carrier is very smooth and the curableliquid composition applied thereto will tend to replicate the surface towhich it is applied, the resulting abrasion-resistant layer will havethe desired high degree of smoothness that provides undistorted opticaltransmission. The thick carrier provides a multi-layer sheet materialthat is resistant to wrinkling, distortion or dimensional change duringthe process of applying the protective laminate to the substraterequiring protection and also provides for ease of stripping after thecarrier has served its purpose. Because of its overall thickness, themulti-layer sheet material can be laminated to the substrate usingrelatively high tension and considerable lamination force. It is verymuch easier to handle than a very thin laminate that does not include astrippable carrier. Thus, for example, with a very thin laminate thatdoes not include a strippable carrier, the degree of tension used in thelaminating step must be very carefully controlled. If there is notenough tension, the result will be formation of wrinkles, whereasexcessive tension can cause curling and distortion of an element such asa phototool.

A wide variety of flexible sheet materials are suitable for use as thestrippable carrier. Examples of suitable materials include polyesters,polycarbonates, polystyrenes, polyvinyl chloride,acrylonitrile-butadiene-styrene resins, polyolefins such as polyethyleneor polypropylene, copolymers of ethylene such as ethylene/vinyl acetatecopolymers, copolymers of vinyl chloride such as vinyl chloride/vinylacetate copolymers, and the like. Particularly preferred materials forthe strippable carrier are polyester films and especially polyethyleneterephthalate film.

The mean thickness of the flexible sheet material used as the strippablecarrier is typically in the range of from about 50 to about 175micrometers and a particularly preferred material has a mean thicknessof about 60 micrometers.

A very important feature of the multi-layer sheet material of thisinvention is the abrasion-resistant layer that is formed by in situcuring of a liquid coating composition. Useful liquid compositions thatare curable to a solid form include heat-curable compositions andradiation-curable compositions that are cured by such means as electronbombardment or exposure to ultraviolet radiation.

The structural interlayer which forms a part of the multi-layer sheetmaterial of this invention can be formed of similar materials to thosethat are used in forming the strippable carrier. A key feature of thestructural interlayer is that it is very thin in relation to thethickness of the strippable carrier. Polyesters and especiallypolyethylene terephthalate are particularly preferred materials.Typically, the mean thickness of the structural interlayer is less than20 micrometers and preferably is in the range of from about 2 to about10 micrometers. Preferably, the mean thickness of the strippable carrieris at least five times, and more preferably at least ten times, that ofthe structural interlayer.

An important function of the structural interlayer is to serve as asupport for the abrasion-resistant layer. Layers which have a highdegree of abrasion-resistance and scratch-resistance, as is required inthe protective laminates of this invention, are typically rather hardand brittle. Hard and brittle layers tend to undergo cracking, whereas asofter material is much less likely to crack but will typically lackscratch- and abrasion-resistance. Therefore, if the abrasion-resistantlayer were employed without the contiguous structural interlayer, therewould be a significant problem of crack formation in theabrasion-resistant layer during application of the protective laminateto the substrate requiring protection. The structural interlayerprovides the tough flexible support that is needed. Additionally, thestructural interlayer provides the means by which the protectivelaminate can be stripped from the surface to be protected and easilyreapplied if a defect occurs during lamination.

A thin adhesive layer overlies the structural interlayer and serves toadhesively bond the protective laminate to the substrate. Usefuladhesives include pressure-sensitive adhesives and adhesives which areactivated by heating, that is, heat-sealable adhesives.Pressure-sensitive adhesives are preferred, and when used, it isnecessary to cover the surface of the adhesive layer with a releasesheet to enable the multi-layer sheet material to be wound in roll formwithout adjacent convolutions of the roll sticking together. Suitablematerials for use as release sheets with pressure-sensitive adhesivelayers are well known in the art. A preferred material for the releasesheet is silicone-coated polyethylene terephthalate film.

The abrasion-resistant layer and adhesive layer can both be very thinlayers, for example, layers with a thickness in the range of from about2 to about 10 micrometers. The thickness of the release sheet is onlyimportant in minimizing the total diameter of a roll of finishedproduct, since this material is removed and discarded. A preferredthickness is about 25 micrometers.

The sheet materials utilized in forming the protective laminates of thisinvention, namely, the carrier, the structural interlayer and therelease sheet, are preferably materials which exhibit a very high degreeof surface smoothness, such as, for example, a surface smoothness in therange of from about 0.01 to about 0.06 micrometers. Additionally, thecoating compositions employed, namely the adhesive composition and thecomposition that is curable to an abrasion-resistant layer, should bothbe coated in the form of very smooth layers of very uniform thickness.These requirements are, of course, dependent on the nature of thesubstrate that requires protection and are especially significant whenthe substrate is a phototool.

In this invention, the protective laminate which serves to protect thesubstrate is composed solely of the abrasion-resistant layer, thestructural interlayer and the adhesive layer, since the strippablecarrier and any release sheet which is employed are removed after theyhave served their purpose. Since each of the abrasion-resistant layer,structural interlayer and adhesive layer is very thin, the combinedstructure can be very thin. This is, of course, highly advantageouswhere it is desired to protect a substrate such as a phototool withoutcreating optical distortions.

The materials employed in preparing the multi-layer sheet material ofthis invention are selected to provide a strong bond between theabrasion-resistant layer and the structural interlayer, since such bondis intended to be permanent. They are selected to provide a weak bondbetween the carrier and the abrasion-resistant layer, and a weak bondbetween the adhesive layer and the release sheet, since these bonds areintended to be temporary in view of the fact that both the carrier andthe release sheet are eventually stripped off and discarded.Advantageously, it will require less stripping force to remove therelease sheet than to remove the carrier since, in use, the releasesheet is typically removed first and the carrier is removed only afterthe laminate has been adhesively bonded to the substrate.

To provide the desired strong bond between the abrasion-resistant layerand the structural interlayer, it is often advantageous to activate thesurface of the structural interlayer by a suitable process, such ascorona discharge treatment, prior to the step of laminating it to thecoated surface of the carrier. Other methods of activating the surfaceare also well known in the art, including various chemical treatmentsand flame treatment. A suitable process of flame treatment is describedin U.S. Pat. No. 3,072,483, while corona discharge treatment isdescribed in numerous patents, including British patents 971 058 and 1060 526 and U.S. Pat. Nos. 2,864,755, 3,117,865, 3,220,842, 3,411,910,3,531,314, 4,298,440 and 4,649,097. Methods of chemical treatment aredescribed in U.S. Pat. Nos. 2,893,896, 3,419,410 and 3,740,252. Toprovide the desired weak bonding to the carrier and the release sheet,it is often advantageous to coat the surface of each of these materialswith a suitable release coating. Useful materials for forming releasecoatings are well known in the art. An example of a particularlyeffective release coating is an organosiloxane.

Polymeric subbing layers used to promote the adhesion of hydrophiliccoating compositions to polyester film supports are very well known inthe photographic art. Useful compositions for this purpose includeinterpolymers of vinylidene chloride such as vinylidenechloride/acrylonitrile/acrylic acid terpolymers or vinylidenechloride/methyl acrylate/itaconic acid terpolymers. Such compositionsare described in numerous patents such as, for example, U.S. Pat. Nos.2,627,088, 2,698,235, 2,698,240, 2,943,937, 3,143,421, 3,201,249,3,271,178, 3,443,950 and 3,501,301. The polymeric subbing layer istypically overcoated with a second subbing layer comprised of gelatinwhich is typically referred to in the art as a "gel sub".

In the present invention, the subbing layers and combinations of subbinglayers referred to above can be used as release coatings which willprovide the desired weak bonding between the abrasion-resistant layerand the strippable carrier. While these compositions are intended topromote adhesion of hydrophilic layers to polyesters, they do just theopposite in the present invention, in that they reduce the strength ofadhesive bonding that would otherwise occur when the hydrophobicradiation-curable composition is cured in contact with the carrier. Asuitable thickness for a release coating of this type on the carriersurface is a thickness of about one micrometer.

FIG. 1 is a cross-sectional view of a protective laminate within thescope of this invention. In this embodiment of the invention, a thinabrasion-resistant radiation-cured layer 12 having a thickness of 4micrometers overlies a thick strippable carrier 10 which is formed ofpolyethylene terephthalate film having a thickness of 60 micrometers. Asindicated by the figure, carrier 10 is releasably bonded to layer 12 andcan be stripped away after it has served its purpose. A thin structuralinterlayer 14, formed of polyethylene terephthalate film having athickness of 5 micrometers, is interposed between layer 12 and a thinoutermost layer 16 composed of a pressure-sensitive adhesive.

FIG. 2 is a cross-sectional view of a modified form of the laminate ofFIG. 1 in which a release sheet 18 overlies adhesive layer 16. As shownin FIG. 2, release sheet 18 is capable of being stripped away, and isordinarily removed and discarded at the time the protective laminate isapplied to a substrate.

FIG. 3 is a cross-sectional view of a modified form of the protectivelaminate of FIG. 1 in which strippable carrier 10 is provided with avery thin release coating 11, formed of a suitable release material suchas an organosiloxane, which facilitates the stripping operation. Asindicated by FIG. 3, stripping occurs at the interface between layers 11and 12.

FIG. 4 is a cross-sectional view of a modified form of the protectivelaminate of FIG. 1 in which the side of strippable carrier 10 oppositeto abrasion-resistant layer 12 is coated with a very thin releasecoating 20 in order to facilitate winding of the protective laminate inroll form.

As explained hereinabove, a key feature of the present invention is acured-in-place abrasion-resistant layer that is releasably bonded by thecuring to the carrier and permanently bonded by the curing to thestructural interlayer. The bond between the carrier and the curedabrasion-resistant layer is sufficiently strong to permit the carryingout of the subsequent manufacturing steps, yet is weak enough for thecarrier to be easily stripped from the abrasion-resistant layer at theappropriate time.

The curable composition that is used in the method of this invention toform the abrasion-resistant layer by an in situ curing technique can beapplied to the strippable carrier by any suitable coating technique thatis effective in applying a very thin layer of uniform thickness. Forexample, the coating step can be carried out by air-knife coating, rollcoating, gravure coating, extrusion coating, bead coating, curtaincoating, use of wire wound coating rods, and so forth.

In the method of this invention, it is particularly preferred to formthe abrasion-resistant layer from a radiation curable coatingcomposition. Such compositions are well known and can be formulated froma very wide range of materials which are curable by radiation to yield asolid cross-linked polymeric matrix. Useful materials of this type canbe monomers, oligomers or polymers, or mixtures thereof. While anyradiation-curable material which can be successfully coated in the formof a very thin layer can be used, compositions comprising an unsaturatedpolymer dissolved in a polymerizable monomer are particularlyadvantageous. It is believed that in these compositions the monomercopolymerizes with the unsaturated polymer to form a cross-linkednetwork. A wide variety of such radiation-curable compositions areknown. For example, classes of polymers which are useful for thispurpose include epoxy diacrylates, unsaturated polyesters, unsaturatedacrylics, unsaturated polybutadienes, unsaturated acrylic modifiedpolyurethanes, unsaturated acrylic modified polythioethers, acrylatedglycols and polyols, unsaturated acrylic-terminated polybutadienes andpolybutadiene/acrylonitriles, and the like. Specific examples of usefulpolymers are an epichlorohydrin/bisphenol-A epoxy that is reacted withacrylic acid or methacrylic acid to form acrylate or methacrylate esterend groups at both ends of the epoxy chain, as well as similar polymersprepared from novolac epoxies (fusible and soluble epoxy resins formedby condensation of a phenol with an aldehyde under acid conditions).Further examples of useful polymers are a bisphenol-A/fumaric acidpolyesters and a di(hydroxypropyl acrylate-anhydride) modifiedbisphenol-A/epichlorohydrin epoxy. Oligomers can be used in theradiation-curable composition in place of or in addition to theaforesaid polymers, for example, a polyoxyethylene diacrylate oligomer.

Further examples of polymeric materials useful in radiation-curablecompositions which are suitably employed in the method of this inventionare described in numerous patents, for example in U.S. Pat. Nos.3,367,992; 3,551,235; 3,554,886; and 3,558,387.

Particularly preferred radiation-curable compositions for the purpose ofthis invention are compositions comprising an acrylated epoxy resin. Theacrylated epoxy resin are well known materials and resins of this typehave been described in numerous patents, for example in U.S. Pat Nos.3,661,576; 3,673,140; 3,713,864; and 3,772,062 and in British Patent No.1,375,177. Typical resins of this type are those derived frombisphenols. In a preferred embodiment of this invention, the acrylatedepoxy resin is a reaction product of epichlorohydrin, bisphenol-A and anacrylic monomer.

When the acrylated epoxy resin is a liquid of a viscosity suitable forcoating, it can be used as the radiation-curable material without theaddition thereto of any polymerizable monomer. When the acrylated epoxyresin is a solid, it can be dissolved in a monofunctional acrylicmonomer to form a composition suitable for coating. Such monomers can,of course, also be used to dilute the resin and render it less viscouswhen it is a liquid of too great a viscosity to be easily coated.Typical examples of monofunctional acrylic monomers useful for thesepurposes are acrylic and methacrylic esters such as ethyl acrylate,butyl acrylate, 2-hydroxypropyl acrylate, cyclohexyl acrylate,2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, and thelike. Polyfunctional acrylates and methacrylates, i.e., those containingat least two acrylic ester groups, can also be advantageously includedin the radiation-curable composition to modify such characteristics ascuring rate and brittleness of the cured layer. Typical examples ofsuitable polyfunctional acrylates and methacrylates are neopentylglycoldiacrylate, trimethylol propane, triacrylate, 1,6-hexanediol diacrylate,1,3-propanediol dimethacrylate, 1,3-butylene glycol dimethacrylate, andthe like. The radiation-curable composition can also contain otherethylenically unsaturated monomers such as styrene, ethyl vinyl benzene,α-methyl styrene, vinyl acetate, and the like. Mixtures of two or moremonofunctional acrylates, of two or more poly-functional acrylates, andof two or more other ethylenically unsaturated monomers can also be usedas desired.

In addition to an acrylated epoxy resin, the radiation-curablecomposition can also contain other resins which serve to modify itsproperties in a desirable manner, for example it can contain resinswhich act to improve its adhesive characteristics. The use of such otherresins is optional and dependent upon the desired characteristics of theproduct.

A further example of a preferred class of radiation-curable materialsfor the purpose of this invention are compositions comprising anacrylated urethane. The acrylated urethanes are well known and commonlyused in radiation-curable coatings. Materials of this type aredescribed, for example, in U.S. Pat. Nos. 3,509,234; 3,600,539;3,694,415; 3,719,638 and 3,775,377 and in British Patent No. 1,321,372.The acrylated urethane resins can be used by themselves or incombination with a different class of resins such as the acrylated epoxyresins.

Apparatus and methods for curing of radiation-curable compositions bysubjecting them to suitable forms of radiation are well known and anysuitable radiation curing process can be used in carrying out thisinvention. For example, curing can be carried out by the application ofultraviolet radiation of suitable intensity. High energy ionizingradiation such as X-rays, gamma rays, beta rays, and acceleratedelectrons can also be used to accomplish curing of the coating.Typically, the radiation used should be of a sufficient intensity topenetrate substantially all the way through the coated layer and will beapplied uniformly to the entire surface of the coated layer. The totaldosage employed should be sufficient to bring about curing of theradiation-curable composition to form a solid plastic. Typically,dosages in the range of about 0.2 to about 50 megarads, more usually inthe range from about 0.5 to about 20 megarads, are employed. Theradiation-curable components of the coating composition are, in mostcases, completely convertible to a solid product.

When the radiation-curable composition is cured by the use ofultraviolet radiation, a photo-initiator should be included in thecomposition. Many photoinitiators which are useful for such purpose areknown to the art, for example, butyl benzoin ether, isobutyl benzoinether, ethyl benzoin ether, propyl benzoin ether, benzophenone, benzilketals, benzoin, acetophenone, dimethyl quinoxiline,4,4'-bis(dimethylamino)benzophenone, and the like. Such photo-initiatorsmay be used singly or in combination. The use of photoinitiators is notnecessary when curing is carried out with high energy electrons.

In a preferred embodiment of this invention, the abrasion-resistantlayer is a layer which also exhibits antistatic properties. Among theadvantages that this provides is a significant reduction in the tendencyto attract dirt during manufacture of the sheet material or during itsapplication to the substrate. Radiation-curable compositions which arecapable of forming an abrasion-resistant antistatic layer are well knownin the art. An example of a suitable composition for this purpose isthat described in Keough, U.S. Pat. No. 4,623,594, issued Nov. 18, 1986,which relates generally to formulations containing a radiation-curableprepolymer and a radiation-reactive antistatic agent which is soluble inthe prepolymer.

In a particularly preferred embodiment of this invention, anabrasion-resistant layer which provides antistatic protection is formedby use of a radiation-curable composition comprising a salt dissolved ina mixture of a poly(alkylene glycol) diacrylate and an acrylic monomercontaining at least three acrylic ester groups. Such compositions aredisclosed in copending commonly assigned U.S. patent application Ser.No. 207,816, filed June 17, 1988 entitled, "Radiation-CurableComposition For Forming An Abrasion-Resistant Antistatic Layer", byJanglin Chen and B. R. Dotson now U.S. Pat. No. 4,957,947. Preferredpoly(alkylene glycol)diacrylates are compounds of the formula: ##STR1##wherein R is an alkylene radical of 2 to 4 carbon atoms, R₁ and R₂ areindependently H or CH₃, an x is an integer having a value of from 3 to50. Particularly preferred poly(alkylene glycol)diacrylates are thepoly(ethylene glycol)diacrylates of the formula: ##STR2## wherein x isan integer having a value of from 5 to 20. The acrylic monomercontaining at least three acrylic ester groups, for example, atriacrylate, tetraacrylate or pentaacrylate, serves to provide enhancedscratch- and abrasion-resistance. Preferred monomers of this type areacrylated pentaerythritols and especially dipentaerythritol monohydroxypentaacrylate. The salt is preferably an alkali metal salt, andespecially an alkali metal fluoroborate. Examples of useful saltsinclude LiBF₄, NaBF₄, KBF₄, Zn(BF₄)₂, KCF₃ SO₃, NaCF₃ SO₃, LiCF₃ SO₃,KCF₃ CO₂, LiCF₃ CO₂, NaC₃ F₇ CO₂, LiI, NaI, KI, KC₄ F₉ SO₃, KPF₆, NaB(C₆H₅)₄, LiClO₄, KSCN, LiSCN, NaSCN, and the like. Typically, theradiation-curable composition contains about 2 to about 20% by weight ofthe salt, about 15 to about 70% by weight of the poly(alkyleneglycol)diacrylate and about 20 to about 80% by weight of the acrylicmonomer containing at least three acrylic ester groups.

In the method of this invention, the curable composition serves severalfunctions. In particular, it serves to form a protective layer which isscratch- and-abrasion-resistant to a greater extent than can be achievedwithout the use of an in situ curing technique, since such cured layerstypically exhibit superior scratch- and abrasion-resistance compared tocast or extruded films. It also serves, in effect, as a laminatingadhesive in the manufacturing process to temporarily bond the structuralinterlayer to the strippable carrier. In preferred embodiments of theinvention, it serves the additional purpose of providing antistaticprotection. Use of the cured-in-place process of this invention to formthe abrasion-resistant layer has the further advantage that it avoidsthe need to dry a liquid layer whose surface is exposed while it passesthrough a drying oven and thus avoids the risk of contamination that canoccur in such a drying process.

The adhesive composition can be applied to the structural interlayer byany suitable coating technique such as the coating methods describedabove in reference to the coating of the radiation-curable composition.Acrylate adhesives which are normally tacky and pressure-sensitive arepreferred.

Any suitable technique can be used to bring the protective laminate intoconforming contact with the substrate requiring protection. In instanceswhere the substrate is a phototool, great care should be exercised tosee that no entrapment of dirt or air bubbles occurs. If an error inalignment or other problem such as formation of a wrinkle occurs in thisstep, the protective laminate can be stripped off, re-positioned andre-applied. The strippable carrier is ordinarily retained to provideprotection until such time as the phototool is put in use.

The essential steps of the manufacturing process are to coat the curableliquid composition on one surface of the carrier; to laminate thestructural interlayer to the wet coated surface; to cure by suitablemeans such as by exposure to radiation through either the carrier or theinterlayer or both; to coat the surface of the structural interlayerwith a layer of adhesive; to apply a release sheet when required; and,under typical conditions, to wind in roll form.

The multi-layer sheet material of this invention is adaptable to use ineither web form or sheet form, as dictated by the requirements of theparticular end use. It is also adaptable to providing a variety of typesof protective surface. For example, a very smooth surface can beachieved by having the curable liquid replicate the surface of a verysmooth strippable carrier. Alternatively, where a matte finish isdesired, as is sometimes the case with color prints, the strippablecarrier can be textured to impart a similar textured surface to theabrasion-resistant layer. The protective laminate can be applied to oneor both faces of a photographic element, for example, it can be appliedover the emulsion layers as a protective overcoat and also on the backside of the film support to protect it from scratch and abrasion damage.

The invention is further illustrated by the following example of itspractice.

A radiation-curable coating composition is prepared as follows:

    ______________________________________                                        Component        Weight %                                                     ______________________________________                                        PEG (400) DA*    54.5                                                         DPEMHPA**        28.3                                                         LiCF.sub.3 SO.sub.3                                                                            11.5                                                         Hydroxy cyclohexyl                                                                             5.7                                                          phenyl ketone***                                                                               100.0                                                        ______________________________________                                         *Poly(ethylene glycol)diacrylate with a molecular weight of approximately     400.                                                                          **Dipentaerythritol monohydroxy pentaacrylate.                                ***A photoinitiator of the benzil ketal class available from CibaGeigy        Corporation under the trademark IRGACURE 184.                            

To form a strippable carrier, a sheet of polyethylene terephthalate filmwith a thickness of 63.5 micrometers is coated with both a polymeric sub(vinylidene chloride/methyl acrylate/itaconic acid terpolymer) and a gelsub, as described in U.S. Pat. No. 3,271,178. The subbed surface of thecarrier exhibits a nominal surface roughness of 0.014 micrometers RootMean Square and a release value of approximately 4 grams/linealcentimeter when peeled at a rate of 200 mm/min from anabrasion-resistant layer formed from the above-describedradiation-curable coating composition.

The radiation-curable coating composition, which exhibits a Brookfieldviscosity of approximately 160 centipoises, is applied as a thin coatingto the subbed surface of the strippable carrier. A suitable coatingtechnique is to employ a reverse gravure coating process with atrihelical patterned gravure cylinder having an engraved volume ofapproximately 9.5 cc/m².

A polyethylene terephthate film with a thickness of 5 micrometers, forexample DuPont's MYLAR film Type C, 24 gauge, is suitable for use as thestructural interlayer. Corona discharge treatment (approximately 0.8joules/cm²) of the surface of this film prior to the wet-lamination stepprovides an adequate level of adhesion to the abrasion-resistant layer.

The corona-discharge-treated surface of the structural interlayer iswet-laminated to the liquid-coated surface of the carrier using a steelroll and an 80 durometer (Shore A) backing roll at a pressure ofapproximately 0.4 Kg/lineal centimeter.

Curing of the layer of radiation-curable coating composition isaccomplished by the use of ultraviolet radiation directed through thestructural interlayer. Medium pressure mercury lamps which emitradiation in the 200-400 nanometer portion of the spectrum are suitablefor this purpose. Curing conditions are adjusted to provide an exposurelevel of 0.5-1.0 joules/cm². The cured layer exhibits excellent scratch-and abrasion-resistance and additionally provides effective antistaticprotection.

After curing of the radiation-curable composition is complete, a thinlayer of adhesive is applied to the surface of the structuralinterlayer. A suitable adhesive for this purpose is anacrylic-resin-based pressure-sensitive adhesive available from Monsantoas GELVA Multipolymer Solution 1753. This adhesive is supplied at anominal concentration of 31% solids in a solvent blend composed of 56weight % ethyl acetate, 27 weight % isopropanol, 10 weight % hexane and7 weight % ethanol. To provide the very thin coatings required in thisinvention, the adhesive solution is further diluted to 8-12% solidsusing a dilution solvent blend composed of 55 weight % ethyl acetate, 35weight % isopropanol and 10 weight % heptane. The resulting solution hasa specific gravity of approximately 0.89 and a Brookfield viscosity ofless than 200 centipoises.

The adhesive is coated directly on the surface of the structuralinterlayer without any prior treatment of the surface. The adhesive iscoated with a fixed-slot hopper to provide a dry coverage of 2.2-8.6grams/m² (preferably 4.3 g/m²). These coverages are roughly equivalentto thicknesses of 2-8 micrometers (preferably 4 micrometers). The levelof adhesion, or degree of peelability, is directly related to the dryadhesive coverage and 4.3±1.0 g/m² provides optimum results. Theconditions under which the adhesive is dried should be such that lessthan one percent total residual solvent remains in the coating prior toapplication of the release sheet. The minimum time and temperaturerequired to achieve this level of dryness is one minute at 70° C.

After the adhesive has been dried, a release sheet is applied to enablethe laminate material to be wound in roll form. A suitable release sheetis POLYSLIK 2002 film, which is available from H. P. Smith PaperCompany, a subsidiary of Phillips Petroleum Company. This material is a25 micrometer thick polyester film with a silicone release coating onone surface. The release surface of this film exhibits a nominal surfaceroughness of 0.036 micrometers Root Mean Square and a release value ofapproximately 3 grams/lineal centimeter when peeled at a rate of 200mm/min from the aforesaid adhesive layer.

In utilizing this invention, a laminate as described above is bonded tothe substrate requiring protection by means of the adhesive coating and,after ensuring that the laminate is correctly positioned, the strippablecarrier is removed to thereby expose the abrasion-resistant layer.

As hereinabove described, a very important feature of the protectivelaminate of this invention is that the strippable carrier is relativelythick, while the structural interlayer is very thin. Many significantadvantages accrue from this relationship of layer thickness in themulti-layer sheet material of this invention. Thus, for example, thethick carrier is easy to convey and handle in the manufacturing process,whereas a very thin sheet material is difficult to convey and handlewithout wrinkling, distortion or other damage. After the curable liquidcomposition is coated on the carrier, the thin structural interlayer islaminated to it by suitable laminating means, such as pressure rollers,and an in situ curing step is then carried out. Thereafter, an adhesivelayer is applied over the structural interlayer. Thus, it is notnecessary to convey the interlayer through a coating station until afterit has been laminated to the carrier, so the severe problems of thin webconveyance and handling over multiple spans that would ordinarily beinvolved with such materials are effectively avoided. Moreover, themulti-layer sheet material of this invention not only has advantages inease of manufacture, but also in ease of use. Thus, it is sufficientlythick and strong when the carrier is present to make it easy toadhesively bond it to the substrate in a very smooth and wrinkle freestate, yet it has the important advantage that it is very thin after thecarrier is removed.

To utilize a very thin protective laminate without employing astrippable carrier, as is done, for example, in U.S. Pat. No. 4,077,830,it is necessary to maintain the laminate under tension while applying itto the phototool so as to avoid the formation of wrinkles. This is acritical and difficult step. If not enough tension is applied, wrinkleswill form; but if too much tension is applied, there is a tendency forexcessive curling of the phototool to occur. To exactly control thetension adds greatly to the difficulty of the process. With the thickstrippable carrier that is utilized in the present invention, thelamination step can be easily carried out, without the need for criticalcontrol of tension and nip forces, and there is no problem of eitherwrinkling of the protective laminate or curl of the substrate.

Use of the laminates of this invention to provide protective surfacesfor substrates requiring abrasion and scratch protection is especiallyadvantageous in the photographic field. Thus, it has been proposed inthe past to apply a radiation-curable liquid composition over thesurface of a processed photographic product, such as a motion picturefilm, and to cure such liquid coating to a solid layer which is scratch-and abrasion-resistant by exposing it to a suitable source of highenergy radiation. (See, for example, U.S. Pat. Nos. 4,092,173,4,171,979, 4,333,998 and 4,426,431). However, this requires thatphotofinishers must carry out the application and curing of theradiation-curable liquid even though they typically do not havepersonnel who are experienced in coating and curing technology. Thislack of experience can result in inconsistent quality and a high levelof waste. In addition, radiation-curable formulations in the liquidstate pose a concern from a health and safety standpoint, as does theradiation-curing equipment. Such concerns are obviated by the presentinvention in that the photofinishing laboratory merely needs to carryout a simple laminating operation using a multi-layer sheet materialthat poses no health or safety concerns, yet the advantages ofradiation-curable formulations with respect to excellent scratch- andabrasion-resistant properties are still achieved.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A multi-layer sheet material that is useful as aprotective laminate; said multi-layer sheet material consistingessentially of:(1) a strippable carrier comprised of a flexible sheetmaterial; (2) an abrasion-resistant layer which overlies said carrierand is releasably boned thereto, said abrasion-resistant layer havingbeen cured by in situ radiation-curing; (3) a thin structural interlayerof flexible laminatable sheet material which overlies saidabrasion-resistant layer and is permanently bonded thereto, as a resultof said in situ radiation-curing of said abrasion-resistant layer, saidinterlayer being very thin in relation to the thickness of said carrier;and (4) an adhesive layer overlying said interlayer; said multi-layersheet material being adapted to be bonded to a substrate by means ofsaid adhesive layer and said carrier being adapted to be stripped fromsaid abrasion-resistant layer to thereby provide a protective laminateon said substrate which has an abrasion-resistant surface.
 2. Amulti-layer sheet material as claimed in claim 1 additionally comprisinga release sheet overlying said adhesive layer.
 3. A multi-layer sheetmaterial as claimed in claim 1 additionally comprising a release coatingon the side of said carrier opposite to said abrasion-resistant coating.4. A multi-layer sheet material as claimed in claim 1 additionallycomprising a release coating interposed between said carrier and saidabrasion-resistant layer.
 5. A multi-layer sheet material as claimed inclaim 1 wherein said interlayer has a mean thickness of less than 20micrometers and said carrier has a mean thickness at least five timesthat of said interlayer.
 6. A multi-layer sheet material as claimed inclaim 1 wherein said interlayer has a mean thickness in the range ofabout 2 to about 10 micrometers, and said carrier has a mean thicknessin the range of about 50 to about 175 micrometers.
 7. A multi-layersheet material as claimed in claim 1 wherein said adhesive layer iscomposed of a pressure-sensitive adhesive.
 8. A multi-layer sheetmaterial as claimed in claim 1 wherein said adhesive layer is composedof a heat-sealable adhesive.
 9. A multi-layer sheet material as claimedin claim 1 wherein said interlayer is composed of polyethyleneterephthalate.
 10. A multi-layer sheet material as claimed in claim 1wherein both said interlayer and said carrier are composed ofpolyethylene terephthalate.
 11. A multi-layer sheet material as claimedin claim 1 wherein said abrasion-resistant layer is also an antistaticlayer.
 12. A multi-layer sheet material that is useful as a protectivelaminate, said multi-layer sheet material comprising:(1) a strippablecarrier of polyethylene terephthalate having a mean thickness in therange of from about 50 to about 175 micrometers; (2) a radiation-curedabrasion-resistant layer which overlies said carrier and is releasablybonded thereto; (3) a structural interlayer of polyethyleneterephthalate having a mean thickness in the range of from about 2 toabout 10 micrometers which overlies said abrasion-resistant layer and ispermanently bonded thereto; (4) a layer of pressure-sensitive adhesiveoverlying said interlayer; and (5) a release sheet overlying saidadhesive layer;said multi-layer sheet material being adapted to bebonded to a substrate by means of said adhesive layer upon removal ofsaid release sheet and said carrier being adapted to be stripped fromsaid abrasion-resistant layer to thereby provide a protective laminateon said substrate which has an abrasion-resistant surface.